The present invention relates to a process for preparing an isoparaffin white oil. It more particularly relates to a process for preparing a low normal paraffin white oil from a refinery stream using an intermediate pore size silicoaluminophosphate molecular sieve.
White mineral oils are highly refined oils which may become part of food either by direct addition or by incidental contact. Two regulations specify properties of food grade white mineral oil:
a. 21 CFR 172,878 governs direct food addition permitted in food for human consumption; and
b. 21 CFR 178.3620(a) governs food additives resulting from contact with containers or equipment.
These two regulations require that the white oil meet test requirements for readily carbonizable substances, for sulfur compounds, and for maximum ultraviolet absorbance limits.
Technical white mineral oils, defined in 21 CFR 178.3620(b), must meet minimum +20 Saybolt color and less stringent ultraviolet absorbance limits using test methods for food grade white oil described above.
A second grade of technical white mineral oil, defined in 21 CFR 178.3620(c) for use as a component of non-food articles, must meet minimum initial boiling point, maximum color and maximum ultraviolet absorbance limits.
Because of the severe requirement for white oils, many commercial processes use purified feed streams in white oil manufacture. For example, International Publication WO 91/08276, filed Dec. 5, 1990, discloses isomerization of isoolefins followed by hydrogenation to form an isoparaffin white oil. Hydrogenation of a polymerized olefin to form white oils is also taught in GB 1,204,012, filed Jul. 31, 1968. A two-stage hydrogen treatment of wax to prepare white oils is taught in GB 1,476,428, filed Jul. 3, 1973.
A number of processes for preparing white oils from petroleum feedstock have also been disclosed. For example, in U.S. Pat. No. 3,553,107, white oils are prepared by subjecting lubricating oil stock first to hydrogenation and then to acid treating using fuming sulfuric acid. GB 1,310,320 describes a three-step process for producing technical white mineral oil from a raw, waxy mineral lubricating oil distillate: (1) a hydrogenation step at 600.degree.-800.degree. F.; (2) a hydroisomerization-hydrocracking step using a crystalline aluminosilicate-containing catalyst; and (3) a hydrogenation step over a platinum group metal hydrogenation catalyst to saturate aromatics and to produce technical white oil.
Recent toxicological research in rats exposed to white oils of varying viscosity and process history in controlled feeding studies demonstrated that hydrocarbons from certain of these oils accumulate in physiologically important organs, including the liver. Physiological changes in the tissues and organs which accumulated hydrocarbons included histiocytosis and granulomatous lesions. Chemical composition and viscosity considerations for the oils tested suggest that the accumulated hydrocarbons may be medium molecular weight n-paraffins. Additional studies in laboratory animals have demonstrated that n-alkanes are apparently more readily absorbed than branched and cyclic alkanes. While the relevance of these studies to humans has not yet been determined, these findings suggest that white oils currently used in the manufacture of foods for human consumption may warrant concern. There is a need, therefore to decrease the normal paraffin content and/or increase the isoparaffin content of white oils.
While isoparaffin white oils have been prepared as noted above, conventional processes for preparing white oils from refinery streams are inadequate for reducing the normal paraffin content of the white oils by selectively isomerizing the normal paraffins to isoparaffins.